1. Field of the Invention
This invention relates to ink-jet printer-plotters, and to a method and device for extending the life of the ink-jet cartridges used in such printer-plotters.
2. Description of the Prior Art
Ink-jet printers, one type being thermal ink-jet printers, are well known in the art, and are commonly used as output devices for printing text and plotting charts in conjunction with personal computers and scientific instruments. Such printers are therefore sometimes called "printer-plotters". Ink-jet printers are made by several companies. Hewlett Packard's family of portable "ThinkJet".RTM. ink-jet printers is a well known example. Thermal ink-jet printing uses thermal excitation (i.e., heat) to fire (i.e., eject) drops (also sometimes called dots) of ink through tiny orifices (i.e., nozzles or jets) in order to print text or graphics such as charts.
A key component of an ink-jet printer is the ink-jet cartridge. The Hewlett Packard thermal ink-jet cartridge is one well known type. It is a disposable unit 20 (see FIG. 1) which contains both ink supply and ink ejection means. The cartridge 20 consists of a liquid ink supply in a bladder 21, twelve ink-ejecting nozzles (i.e., jets) 22a, 22b, etc., and twelve corresponding thin film resistors (not shown) for applying ink selectively to the nozzles. The resistors are located directly below each jet 22a, 22b, etc. An ink-drop ejection process begins by heating the resistor of a selected ink-jet with a short electrical pulse. Within a few microseconds, the ink above the resistor is vaporized. The vapor bubble grows rapidly and imparts momentum to nonvaporized ink above the bubble. Some of this nonvaporized ink is ejected through the jet orifice 22a at velocities exceeding ten meters per second. The jet 22a is then automatically refilled with ink by capillary action.
The ink supply is contained in a synthetic rubber bladder 21 located immediately behind a printhead substrate 23. The bladder 21 is designed to maintain a relatively constant back pressure at the jets 22a, 22b, 22c, etc. which is high enough to refill the jets after firing but low enough so ink is only expelled when desired.
The cartridge 20 also includes locating pins 24, cover 25, resistor array electrical contacts 26a, 26b, etc., and body 27.
Printers using cartridges of this type are typically controlled by a computer program installed in ROM (Read Only Memory) of a microcontroller in the printer. This computer program (i.e., software) conventionally accepts information from a host computer of scientific instrument and controls the printing of text or plotting of graphic images by the printer.
When conventionally used for plotting graphs, charts, etc., in scientific, industrial, and similar applications, the printer is operated in "plot mode" and in this plot mode only one of the twelve jets is used.
This heavy use of one jet in plot mode puts a disproportionate amount of "stress" on that one jet. Such stressing of one jet can result in higher failure rates of the cartridge. It is believed that there is more than one failure mechanism at work in what is termed "stress". First, there is the cycling of the resistor associated with the one jet used in plot mode which vaporizes the ink. The thermal cycling provides a higher than desired failure rate due to fractures in the resistor which can occur by way of the thermal expansion and contraction of the resistor.
Second, there is a mechanism described in the Hewlett-Packard Journal of May 1985 (page 32) as "kogation". Kogation is "the plaque buildup found on the resistors of a thermal ink-jet head after several firings of the system. It can cause the head to fail by insulating the resistor from the ink supply, which reduces bubble generation. The major source of kogation is the ink. By modifying the ink material appropriately, kogation can be varied from rapid buildup to excessive erosion. In the ThinkJet ink, formulas have been optimized to provide a very slow buildup of plaque."
Thus the prior art method of plotting tends to cause failure of the one jet used for plotting, due to thermal cycling and kogation on that one jet. This failure of one jet renders the entire cartridge useless for printing or plotting well before all the ink in the cartridge has been used. Thus the prior art method of plotting causes cartridge failure resulting in extra expense and possible loss of valuable plot data due to cartridge failure.